The long term goals of this research are to more fully understand the autonomic involvement exhibited by subjects who show excessive cardiac output responses relative to metabolic demand in certain behavioral challenges. This work seeks to achieve this goal by focusing on parasympathetic nervous system (vagal) and respiratory responses to behavioral stressors such as reaction time and mental arithmetic as well as more passive tasks such as the cold pressor. This focusing will be in addition to previously established indexing of sympathetic nervous system influences. Subjects will be normotensive young (18-22 years) males and will have a variety of physiological responses monitored, as well as demographic variables such as parental health history and subject exercise habits. This research is important in trying to further define the mechanisms of cardiovascular control seen in behavioral stress, with our excessive reactors providing a model for hemodynamic patterns seen in many young borderline hypertensives. The role of parasympathetic and respiratory influences will be assesed by examining the relationship of breathing patterns and subsequent respiratory sinus arrhythmia to the behavioral challenges. Coupled with this focus, end tidal O2 and CO2 as well as sympathetic influences will be measured to more fully understand dual autonomic influences both at rest and during challenging tasks. To accomplish this, the following cardiopulmonary variables will be measured noninvasively: cardiac output, stroke volume, heart rate, blood pressure, systolic time intervals, forearm blood flow, tidal volume, respiratory rate, minute ventilation, end tidal O2 and CO2, O2 consumption, and CO2 production. It is also a specific aim of this proposal to further evaluate the relationship of breathing patterns and cardiovascular response by controlling minute ventilation patterns to examine cardiopulmonary outcomes with our low and high cardiac output reactors. We feel this has importance for our understanding of respiratory-cardiovascular interaction in our experimental paradigms.